Dual nitrogen expansion process

ABSTRACT

A method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream under-going a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams. The liquefaction can occur in three stages: in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas; in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas.

The present invention relates to a natural gas liquefaction process in which nitrogen is used as the main refrigeration component. The process is particularly, but not exclusively, suited for use offshore.

Natural gas can be obtained from the earth to form a natural gas feed which must be processed before it can be used commercially. The gas is often liquefied before being transported to its point of use. This enables the volume of the gas to be reduced by about 600 fold, which greatly reduces the costs associated with storing and transporting the gas. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. At atmospheric pressure, the usual temperature range within which liquefaction occurs is between −165° C. and −155° C. Since the critical temperature of natural gas is about −80° C. to −90° C., the gas cannot be liquefied purely by compressing it. It is therefore necessary to use cooling processes.

It is known to cool natural gas by using heat exchangers in which a gaseous refrigerant is used. One known method comprises a number of cooling circuits, typically three, in the form of a cascade. In such cascades, refrigeration may be provided by methane, ethane and propane, or other hydrocarbons, with each cycle of the cascade operating at a lower temperature than the last.

In each cycle cool, compressed refrigerant is expanded, causing further cooling, and then fed into a heat exchanger where it is placed in indirect contact with the natural gas. Heat from the natural gas warms and often vaporises the refrigerant, thus cooling the natural gas. The heated refrigerant exits the heat exchanger and is then compressed and cooled, whereupon the cycle is repeated. Often the compressed refrigerant is cooled within the same heat exchanger as the natural gas, i.e. the compressed refrigerant is cooled by the same refrigerant in expanded form.

In a cascade system, in addition to cooling the natural gas, each cycle is also used to cool the refrigerants of the later, cooler refrigeration cycles. This cooling can take place in the same heat exchanger as the cooling of the natural gas or in a separate heat exchanger.

A cascade arrangement which uses mixed refrigerant streams is described in WO 98/48227.

It will be appreciated that the use of hydrocarbons as refrigerants poses a safety issue and this is particularly significant in the offshore environment, where it is highly undesirable to have large liquid hydrocarbon inventories in what is inevitably a confined space.

Several systems have been proposed in which carbon dioxide acts as a refrigerant fluid. For example, U.S. Pat. No. 6,023,942 discloses a natural gas liquefaction process in which carbon dioxide may be used as a refrigerant. This process is not suitable however for large scale or offshore applications, since it relies not on a cascade arrangement but on an open loop expansion process as the primary means of cooling the LNG (liquefied natural gas) stream. Expansion processes such as this do not allow sufficiently low temperatures to be attained, and hence the LNG has to be kept at very high pressures to maintain it in liquid form. Both from a safety and an economic point of view, these high pressures are not suitable for industrial production of LNG and particularly not for large scale or offshore applications.

US 2003/0089125 discloses the use of carbon dioxide within a closed loop cascade system to pre-cool the natural gas. While this pre-cooling circuit reduces the amount of hydrocarbon refrigerant required, hydrocarbons are still used in the subsequent liquefaction and sub-cooling cycles. This is because carbon dioxide cannot be cooled to low enough temperatures to fully liquefy natural gas without solidifying.

Another known alternative is to use a nitrogen refrigerant in a gas expansion process. Traditionally this has the disadvantage that the thermal efficiency of nitrogen is much lower than in a hydrocarbon based system. In addition, because a gaseous refrigerant has a low heat transfer coefficient compared to an evaporating refrigerant, a large heat transfer area is required to dissipate the waste heat from the process into a cooling medium.

U.S. Pat. No. 6,446,465 discloses a liquefaction process using nitrogen in which two separate streams of nitrogen are, used to liquefy the natural gas. The two refrigeration streams are compressed and cooled whereupon one of these streams is fed through a heat exchanger where it is cooled, together with the natural gas, which has already been pre-cooled within a separate pre-cooling system. The cooled nitrogen stream is then expanded to lower its temperature further and is used within a second heat exchanger to further cool the gas. The second nitrogen stream meanwhile is expanded to the same pressure as the first nitrogen stream and combined with the first stream upon its exit from the second heat exchanger. The combined first and second refrigerant streams are then introduced back into the first heat exchanger to provide cooling to the natural gas and first, compressed nitrogen stream. As the natural gas is pre-cooled prior to reaching the nitrogen refrigeration circuit the power requirements of this circuit are significantly reduced. In addition, by feeding the second refrigerant stream directly to the expansion means without passing through the first heat exchanger the heat transfer area in the first heat exchanger is reduced.

US 2005/0056051 discloses a further liquefaction system in which a nitrogen refrigeration circuit is used to cool at least partially liquefied natural gas. The natural gas is pre-cooled and substantially liquefied by a separate circuit in which hydrocarbons are used as the refrigerant. This substantially liquefied natural gas is then fed to the nitrogen cooling system for further cooling. In this document a number of configurations for the nitrogen refrigeration circuit are disclosed, all of which involve a first heat exchanger, in which expanded, low pressure nitrogen cools the LNG, and a second heat exchanger in which the warmed, expanded nitrogen from the first heat exchanger is used to cool the compressed high pressure nitrogen prior to expansion. In several of these embodiments the first and second nitrogen streams are expanded to different pressures.

Therefore no system currently exists for providing complete liquefaction of LNG without the use of hydrocarbons. Further, there is a need within the industry to provide a nitrogen refrigeration system having reduced complexity, simple operation and higher efficiency. Such a system would provide large benefits, particularly in relation to offshore LNG production.

In accordance with one aspect of the present invention there is provided a method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams.

Viewed from another aspect the present invention provides a natural gas liquefaction apparatus comprising one or more heat exchangers for placing the natural gas in a heat exchanging relationship with first and second nitrogen refrigerant streams; one or more compressors for compressing the first and second nitrogen refrigerant streams; a first expander for expanding the first nitrogen stream to a first pressure and a second expander for expanding the second nitrogen stream to a second, lower pressure; wherein the apparatus is arranged such that, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams.

Therefore, in the present invention the nitrogen is expanded to two different pressures. In comparison to a single expansion process the use of two expanders reduces the volume of nitrogen which must be expanded to and compressed from the lowest pressure and therefore reduces the required size and power consumption of these components.

Both the first, intermediate pressure, and second, lower pressure, nitrogen streams are used to cool the natural gas as well as the compressed nitrogen streams prior to their expansion. Unlike prior art systems the natural gas is cooled within each nitrogen heat exchanger. In addition, in at least one heat exchanger the expanded nitrogen streams are also used to cool the compressed streams. This ensures that the maximum amount of heat exchange occurs between the refrigerant and the natural gas and allows this system to operate independently, i.e. without the need for additional heat exchanger circuits to cool the natural gas.

Preferably the liquefaction method comprises three cooling stages. These consist of a final cooling stage, in which the second nitrogen stream is expanded to a low pressure and placed in heat exchanging relationship with the natural gas, an intermediate stage, in which the first nitrogen stream is expanded to an intermediate pressure and placed in heat exchanging relationship with the natural gas to cool the natural gas prior to the final stage, and an initial cooling stage in which the first and second nitrogen streams, after undergoing heating in the final and/or intermediate stages, are placed in heat exchanging relationship with the natural gas to cool this prior to the intermediate stage.

Preferably the second nitrogen stream is also used within the intermediate stage, after heating in the final stage; to provide further cooling to the natural gas. Preferably the intermediate stage also provides cooling for the compressed second nitrogen stream, prior to its expansion and use in the final stage.

The use of three nitrogen stages to cool natural gas is considered inventive in its own right and therefore, viewed from a further aspect the present invention provides a natural gas liquefaction method in which natural gas is cooled by first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, the method comprising three stages in which:

in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas;

in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and

in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas.

Preferably the heated expanded second nitrogen stream is also used to cool the natural gas within the intermediate stage. Preferably the compressed second nitrogen stream is cooled in the intermediate stage.

By re-using the heated, expanded (low pressure) nitrogen streams to contribute to earlier cooling stages this system can operate independently, without the need for an additional pre-cooling or liquefaction circuit. However, in some embodiments, the system can also be used with a pre-cooler which applies external pre-cooling (preferably in the range of 0° C. to −60° C.) of the natural gas, and preferably also the nitrogen streams. Although the use of a pre-cooler increases the complexity of the system, it reduces the energy consumption of the system.

As only one refrigeration circuit is required the liquefaction apparatus is greatly simplified.

Although described as independent streams, the first and second nitrogen streams do not always need to be separate. It is possible for example for the first and second compressed streams to be combined during the first stage and during compression. It is only necessary for the streams to be transported separately through the system at those stages of the cycle in which the streams are at different pressures.

Preferably, in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas. This increases the efficiency of the process.

Preferably, the expanded first nitrogen stream is compressed from the intermediate pressure after cooling the natural gas in the initial and intermediate stages. This reduces the energy required at the compression stage of the process, since only the expanded second nitrogen stream must be compressed from the lower pressure to a higher pressure. This is an improvement over existing processes, in which the entire refrigerant must be compressed from the lower pressure.

By intermediate pressure it is meant any pressure lower than that of the compressed nitrogen stream but greater than that of the expanded second refrigerant stream. Preferably the intermediate pressure is in the range of 15-25 bar. By low pressure it is meant any pressure lower than the intermediate pressure. Preferably the low pressure is in the range of 5-20 bar.

The stages described by the present invention can each occur in a single heat exchanger or multiple heat exchangers. Alternatively, one or more stages can be combined in a single heat exchanger and it is possible for all three stages to occur within a single heat exchanger. Therefore the cooling stages are not defined by heat exchangers but by the nitrogen streams which are used to provide cooling.

Preferably the first nitrogen stream, which is expanded to a first intermediate pressure, comprises a larger volume of nitrogen than the second nitrogen stream. This significantly reduces the volume of nitrogen being expanded to low pressure and thus reduces the power requirements of the low pressure expander. This also reduces the power requirements of the low pressure compressor, or the first stage of a multistage compressor, which is used to compress the expanded second refrigerant stream.

In a preferred embodiment the natural gas liquefaction method provides complete cooling of the natural gas. That is the natural gas does not need to undergo any pre-cooling or partial liquefaction prior to cooling by the present invention. Instead the nitrogen refrigerant streams provide all necessary cooling to the natural gas from ambient temperature to storage temperature. This simplifies the liquefaction system.

By pre-cooling it is meant cooling of the natural gas flow to a temperature at which liquefaction of C₃ components starts to occur. This allows these heavier components to be separated out of the natural gas stream prior to further cooling. This is advantageous as otherwise these heavier components may “freeze out” during liquefaction and impede the flow of natural gas. Typically the pre-cooling phase of a natural gas liquefaction process cools the gas to a temperature of approximately −50° C.

By subcooling is meant cooling of the condensed liquefied gas below the bubble point temperature.

In such “complete cooling” embodiments the outlet temperature of the natural gas from the initial cooling stage is typically between −10° C. and −30° C. Following passage through the intermediate stage, in which the first nitrogen stream is expanded, the outlet temperature is typically between −70° C. and −90° C. Following the final stage, in which the second nitrogen stream is expanded, the outlet temperature of the natural gas is typically in the range of −140° C. to −160° C. This allows the natural gas to be stored and transported in a liquid state without the requirement for this to be pressurised. However, the methods and the apparatus in accordance with the present invention can produce liquid natural gas at elevated pressure (1 to 20 bar) with a corresponding temperature of −100° C. to −165° C.

Preferably the method further includes the step of removing C₅₊ hydrocarbons from the natural gas. Most preferably C₃₊ hydrocarbons are removed. This prevents these heavier hydrocarbons from “freezing out” during liquefaction. This separation step preferably occurs in between the pre-cooling and liquefaction phases. Most usually this point occurs within the intermediate cooling stage however in some embodiments separation may take place between the initial and intermediate stages.

The separation step is carried out by a heavy hydrocarbons removal column. Such columns are well known in the art. As stated above, the exact location of the heavy hydrocarbon column (HHC) will depend on the temperature of the natural gas at differing points within the process.

Preferably the first and second nitrogen streams are compressed in a three stage compression process. This can be provided by individual compressors or by a multi-stage compressor. In the initial compression stage the expanded second nitrogen stream is compressed to the intermediate pressure of the expanded first nitrogen stream and then cooled, preferably by sea water or air. The second compressor stage compresses both the partially compressed second nitrogen stream and the expanded first nitrogen stream. The final compression stage compresses both first and second nitrogen streams. Any of these stages can be provided by two or more parallel compressors. Such an arrangement enables the first and second expanders to drive the third compressor stage, which makes the system more efficient. Alternatively a two stage compression process can be provided.

Viewed from a further aspect the present invention provides a natural gas liquefaction apparatus arranged to carry out the method of the present invention.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures in which:

FIG. 1 shows a natural gas liquefaction process comprising two nitrogen refrigerant circuits in accordance with the present invention;

FIG. 2 shows a further embodiment of the present invention in which a heavy hydrocarbon column is used.

FIG. 1 shows a natural gas liquefaction system 100. Feed gas enters the system via line 1. This gas could be at ambient temperature or pre-cooled via heat exchange with air or water. The feed gas is successively cooled by a number of heat exchangers 101, 102, 103 such that sub-cooled liquefied natural gas exits the last heat exchanger 103 via line 4. This LNG is expanded to atmospheric pressure by expansion valve 109 and fed via line 5 into separator column 110. LNG exits this column 110 via bottom stream 6 while any remaining gaseous elements are removed via line 7 for further cooling.

The natural gas is cooled within heat exchangers 101, 102, 103 by first and second nitrogen streams 121, 122. These streams 121, 122 are cooled collectively by a compression system 108, which will be described later. The combined streams exit the compressor system via line 22 and are then split prior to entry into the first heat exchanger 101. Both nitrogen streams 121, 122 are cooled within this heat exchanger 101. The first nitrogen stream 121 exits the heat exchanger via line 24 and is expanded to an intermediate pressure by expander 111. The expanded first nitrogen stream 121 is then fed via line 25 into second heat exchanger 102 as a cooling fluid. The expanded first nitrogen stream 121 cools both the natural gas and the second nitrogen stream 122 within this heat exchanger 102. Upon exit of the second heat exchanger 102 the warmed expanded first nitrogen stream 121 is re-introduced into the first heat exchanger 101 via line 26. Here it again acts to cool the natural gas as well as the compressed first and second nitrogen streams 121, 122. The warmed, expanded nitrogen stream 121 is then directed through line 27 to the compressor system 108.

After cooling in the first heat exchanger 101 compressed second nitrogen stream 122 is fed via line 29 into, the second heat exchanger 102 for further cooling. After exiting the second heat exchanger 102 the compressed second nitrogen stream 122 is fed via line 30 into second expander 112 for expansion to a pressure lower than the intermediate pressure provided by expander 111. The expanded, cooled second nitrogen stream 122 is then fed via line 31 into third heat exchanger 103 for heat exchange with the natural gas. Upon exit from the third heat exchanger 103 the warmed, expanded second nitrogen stream is fed into the second heat exchanger 102 via line 32 to assist in cooling the natural gas and compressed second nitrogen stream 122 before finally being fed into the first heat exchanger 101 by line 33 to assist in the initial stage cooling of the natural gas and first and second compressed nitrogen streams 121, 122. Following exit from the first heat exchanger 101 the warmed, expanded second nitrogen stream 122 is returned to the compressor system 108 by line 10.

In the present embodiment the compressor system 108 comprises three compressor stages. The first compressor stage comprises a single compressor 113 which compresses the low pressure warmed second nitrogen stream 122, delivered by line 10 from the first heat exchanger 101. The partially compressed second nitrogen stream is then combined with the warmed intermediate pressure first nitrogen stream 121 provided byline 27. The combined first and second nitrogen streams 121, 122 are then further compressed in second stage compressor 114. The last compressor stage comprises two compressors 115 a, 115 b which operate in parallel and are driven by expanders 111, 112. The combined first and second nitrogen streams 121, 122 are split into lines 16, 19 and compressed within the third stage compressors 115 a, 115 b. The splitting of combined nitrogen streams 121, 122 at this point does not necessarily result in all of first nitrogen stream passing through one third stage compressor while the second nitrogen stream passes through the other. Instead, the compressed streams can be split in any ratio at this point. In between each compression stage the nitrogen is cooled by heat exchangers 116, 117, 118 a and 118 b. After final stage compression the combined nitrogen streams are returned to line 22 for separation and re-introduction into the first heat exchanger 101.

In the above described embodiment of the present invention nitrogen streams 121 and 122 provide all necessary cooling to the natural gas. The first heat exchanger 101 cools the natural gas to between −10° C. and −30° C., the second heat exchanger 102 to between −70° C. and −90° C. while the final heat exchanger 103 cools the natural gas to between −140° C. to −160° C. Expander 111 typically expands the first nitrogen stream 121 to a pressure of 15-20 bar while expander 112 typically expands the second nitrogen stream 122 to a pressure of 5-20 bar. The first and second nitrogen streams 121, 122 do not contain the same volume of nitrogen. Instead the largest flow is found in the first nitrogen stream 121. This reduces the power requirements of the low pressure expander 112 and first stage compressor 113.

The use of the warmed expanded nitrogen streams 121, 122 to continue to provide cooling within the earlier heat exchangers ensures that the system is efficient and allows the nitrogen to provide complete cooling without relying on any other refrigeration means to provide liquefaction, partial liquefaction or pre-cooling.

FIG. 2 shows a very similar refrigeration system 200 to FIG. 1. Identical components have been indicated by use of the same reference numerals. Again first and second nitrogen streams 121, 122 are cooled collectively by compression system 108 and further cooled within heat exchanger 101. The first refrigerant stream 121 is then sent via line 24 to expander ill and expanded to an intermediate pressure. The expanded nitrogen is then fed via line 25 into the intermediate, or second cooling stage. Unlike the system of FIG. 1, in this system 200 the intermediate cooling stage occurs in two separate heat exchangers 202 a, 202 b. In both of these heat exchangers 202 a, 202 b the expanded first refrigerant stream 121 and the warmed, expanded second refrigerant stream 122 exiting heat exchanger 103 are used to provide cooling to the natural gas and the compressed second nitrogen stream 122.

In between heat exchangers 202 a, 202 b the natural gas is fed via line 2 a into a Heavy Hydrocarbon Column (HHC) 219. This separates the heavier components, such as C₃₊, from the natural gas stream. These heavier components are removed via line 8 while the natural gas stream is fed via line 2 b to heat exchanger 202 b to continue the cooling process. The natural gas is diverted and fed through the HHC 219 at a stage in the cooling process at which pre-cooling has occurred but prior to liquefaction. Removing heavier hydrocarbons at this stage prevents these from “freezing out” during later parts of the cooling process.

The second nitrogen stream 122, which is cooled in the intermediate stage, is not fed through the HHC 219 but is transferred straight from the heat exchanger 202 a to heat exchanger 202 b via line 29 a. Similarly, the expanded first refrigerant stream and warmed, expanded second nitrogen stream are transported via lines 25 a and 32 a respectively directly between the heat exchangers 202 a, 202 b.

The remainder of the process 200 is identical to that of process 100. 

1. A method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams—is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams.
 2. A method as claimed in claim 1 wherein the liquefaction method comprises three cooling stages; a final cooling stage, in which the second nitrogen stream is expanded to a low pressure and placed in heat exchanging relationship with the natural gas, an intermediate stage, in which the first nitrogen stream is expanded to an intermediate pressure and placed in heat exchanging relationship with the natural gas to cool the natural gas prior to the final stage, and an initial cooling stage in which the first and second nitrogen streams, after undergoing heating in the final and/or intermediate stages, are placed in heat exchanging relationship with the natural gas to cool this prior to the intermediate stage.
 3. A method as claimed in claim 2 wherein the second nitrogen stream, after being heated in the final stage, is used within the intermediate stage to provide further cooling to the natural gas.
 4. A method as claimed in claim 2 wherein the intermediate stage provides cooling for the compressed second nitrogen stream, prior to its expansion and use in the final stage.
 5. A natural gas liquefaction method in which natural gas is cooled by first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, the method comprising three stages in which: in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas; in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas.
 6. A natural gas liquefaction method as claimed in claim 5 wherein, in the intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used, together with the heated expanded second nitrogen stream, to cool the natural gas and the compressed second nitrogen stream.
 7. A natural gas liquefaction method as claimed in claim 5 wherein the first and second compressed streams are combined during the initial stage and during compression.
 8. A natural gas liquefaction method as claimed in claim 5, wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas.
 9. A natural gas liquefaction method as claimed in claim 5, wherein the expanded first nitrogen stream is compressed from said intermediate pressure after cooling the natural gas in the initial and intermediate stages.
 10. A method as claimed in claim 1 wherein the first nitrogen stream comprises a larger volume of nitrogen than the second nitrogen stream.
 11. A method as claimed in claim 1 wherein the first and second nitrogen streams are compressed in a three stage compression process.
 12. A method as claimed in claim 1 further comprising the step of removing C₃₊ hydrocarbons from the natural gas after pre-cooling.
 13. A method as claimed in claim 1 wherein the method provides complete cooling of the natural gas.
 14. A natural gas liquefaction apparatus comprising one or more heat exchangers for placing the natural gas in a heat exchanging relationship with first and second nitrogen refrigerant streams; one or more compressors for compressing the first and second nitrogen refrigerant streams; a first expander for expanding the first nitrogen stream to a first pressure and a second expander for expanding the second nitrogen stream to a second, lower pressure; wherein the apparatus is arranged such that, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams.
 15. A natural gas liquefaction apparatus arranged to carry out the method of claim
 1. 16. A natural gas liquefaction apparatus as claimed in claim 14 wherein the apparatus is arranged to provide complete cooling of the natural gas.
 17. A method as claimed in claim 3 wherein the intermediate stage provides cooling for the compressed second nitrogen stream, prior to its expansion and use in the final stage.
 18. A natural gas liquefaction method as claimed in claim 6 wherein the first and second compressed streams are combined during the initial stage and during compression.
 19. A natural gas liquefaction method as claimed in claim 6 wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas.
 20. A natural gas liquefaction method as claimed in claim 7 wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas. 